In the field of image forming apparatuses, a fixing device having a fixing belt that can reduce the heat capacity of the fixing device has been attracting attention. In recent years, an electromagnetic induction heating type fixing device capable of rapid heating and high-efficiency heating has been attracting attention.
In an electromagnetic induction heating type fixing device, it is advantageous to suppress an excessive temperature rise in a heating rotating body in regions on the outer side of a paper-passing region through which paper passes (non paper-passing regions) according to the width of paper as a receiving material transported to (passed through) the fixing device (the width of paper in a direction perpendicular to the paper transport direction: paper passing width). For this purpose, there has been proposed a technology concerned with a fixing device that adjusts the amounts of heat generation of the heating rotating body in the non paper-passing regions and the paper-passing region.
The fixing device in the proposed technology includes a heating rotating body, a pressing rotating body, an induction coil that generates magnetic flux, a magnetic core portion, a magnetic flux blocking member that reduces or blocks the magnetic flux generated by the induction coil, and a moving mechanism that moves the magnetic flux blocking member. In the technology, an excessive temperature rise in the non paper-passing regions of the heating rotating body can be suppressed by moving the magnetic flux blocking member corresponding to the paper passing width of paper to be passed and adjusting the amount of magnetic flux passing through the magnetic core portion.
However, in the above mentioned fixing device, the temperature of the non paper-passing regions sometimes rises excessively when small size paper is passed, and image offset sometimes occurs in the formed image when printing is performed on large size paper after continuous printing on small size paper. In the case in which adjustment is performed so that the temperature of the non paper-passing regions of small size paper does not rise excessively, when the temperature of the non paper-passing regions of the small size paper becomes too low after continuous printing on small size paper, unsatisfactory fixation may occur when large size paper is passed after that. This problem can also occur when in the above fixing device, in place of the heating rotating body, a heating rotating belt that can reduce the heat capacity of the fixing device is used.